The citygen application is usable at the moment but I'm only beginning to work with it so much better results should be possible in a few weeks.
The primary road network roads are terrain adaptive and plotted to optimum routes around hills, etc. Within enclosed regions of the primary road network, secondary roads are generated using a parametric organic growth algorithm. I've submitted an abstract for a short paper with a little more detail hope to get that up soon. Let me know what you think of the app.